1. Field of the Invention
The present invention relates to an automatic transmission for use in automotive vehicles, and more specifically to a hydraulic control system for such a transmission which features a valve arrangement which improves the rapidity with which engine braking can be produced in response to a manually induced signal to inhibit the top speed of the transmission.
2. Description of the Prior Art
THM700- R4 PRINCIPLES OF OPERATION discloses an automatic transmission arrangement which can produce four forward speeds. In this instance the fourth speed is arranged to produce an overdrive.
This system includes a manual valve which can be selectively moved between P, R, N, D and 3rd, 2nd and 1st manual ranges. When the manual valve is set in D range the transmission is able to automatically shift between the first and fourth speeds, when conditioned to establish the manual third speed range, fourth speed is inhibited and the transmission is permitted to shift only between first and third speeds. Viz., in this condition overdrive is inhibited. When the 3-4 shift valve of this arrangement assumes its 3rd speed or downshift position, a band brake of the transmission is released and at the same time the hydraulic pressure from the manual valve is transmitted via the 4-3 sequence valve to an overrunning clutch. Under these conditions engine braking is produced in speeds below fourth gear.
However, this arrangement has suffered from the drawback in that a delay occurs between the time the manual valve is moved into the manual third speed range position and the time wherein the overrunning clutch actually becomes engaged. That is to say, when the manual valve is moved from D range to the manual third speed range, the line pressure which is supplied from the oil pump to the manual valve has to pass through the conduits interconnecting the manual valve, the 4-3 sequence valve, and the overrunning clutch. This defines a relatively long pressure transmission path which delays the supply of pressure and results in the overunning clutch not being engaged as quickly as preferred. Accordingly, for a period following the D-3 select, engine braking is not produced and deteriorates both the response characteristics of the transmission and the driver's sense of control over the transmission.